Noise detector and suppressor for magnetic resonance imaging equipment

Abstract

An improved transient interference detector and suppressor for an MRI system infers the presence of transient interference in a desired frequency range (in-band signals) by comparing the amplitude of near-band signals to an expected noise reference level. If the near-band signals exceed the expected noise reference level, the presence of transient interference in the in-band signals is concluded. The detector and suppressor may include an automatic adjustment circuit for automatically adjusting the expected noise reference level based on measured statistical information. The detector and suppressor may also require the near-band signals to exceed the expected noise reference level at least twice in a predetermined period before the presence of interference is inferred. A voltage limiter may also be applied to the near-band signals prior to comparison with the expected noise reference level.

The embodiments of the present invention in which an exclusive property or privilege is claimed are defined as follows:  
     
         1 . A method for detecting transient interference in a signal received from an MRI receiving antenna comprising: 
 passing said signal through a voltage limiter to produce a limited signal;    passing said limited signal through a filter designed to remove frequencies within said signal that are within a desired frequency bandwidth to produce a limited, filtered signal;    comparing said limited, filtered signal to a noise reference level; and    determining that said signal likely includes interference if said limited, filtered signal exceeds said noise reference level.    
     
     
         2 . The method of  claim 1  further including automatically adjusting said noise reference level.  
     
     
         3 . The method of  claim 2  wherein said automatic adjustment of said noise reference level includes taking statistical measurements of said limited, filtered signal and generating a threshold signal that has a probability of exceeding said limited, filtered signal a predetermined amount.  
     
     
         4 . The method of  claim 1  further including determining if said limited, filtered signal exceeds said noise reference level at least twice during a predetermined time period and concluding that said signal likely includes transient interference only if said limited, filtered signal exceeds said noise reference level at least twice during said predetermined time period.  
     
     
         5 . The method of  claim 4  further including determining an expected frequency component of said transient interference and setting said predetermined time period to be greater than twice said expected periodicity.  
     
     
         6 . The method of  claim 4  wherein said predetermined time period is on the order of 10 to 100 nanoseconds.  
     
     
         7 . The method of  claim 1  further including blanking said signal if said limited, filtered signal exceeds said noise reference level, and not blanking said signal if said limited, filtered signal does not exceed said noise reference level.  
     
     
         8 . The method of  claim 1  further including generating a re-scan signal if said limited, filtered signal exceeds said noise reference level.  
     
     
         9 . A method for detecting transient interference in a signal received from an MRI receiving antenna comprising: 
 passing said MRI antenna signal through a filter designed to remove frequencies within said signal that are within a desired frequency bandwidth to produce a filtered signal;    comparing said filtered signal to a noise reference level; and    determining that said MRI antenna signal likely includes transient interference if said filtered signal exceeds said noise reference level at least twice within a predetermined time period.    
     
     
         10 . The method of  claim 9  further including passing said signal through a voltage limiter prior to passing said signal through said filter.  
     
     
         11 . The method of  claim 9  further including automatically adjusting said noise reference level.  
     
     
         12 . The method of  claim 11  further including automatically adjusting said noise reference level based on the statistical distribution of the filtered signal.  
     
     
         13 . The method of  claim 11  further including passing said signal through a voltage limiter prior to passing said signal through said filter.  
     
     
         14 . The method of  claim 9  wherein said predetermined time period is on the order of 10-100 nanoseconds.  
     
     
         15 . The method of  claim 9  further including blanking said signal if said filtered signal exceeds said noise reference level, and not blanking said signal if said filtered signal does not exceed said noise reference level.  
     
     
         16 . The method of  claim 9  further including generating a re-scan signal if said filtered signal exceeds said noise reference level  
     
     
         17 . A transient interference detector system for detecting transient interference in an MRI signal received from a receiving antenna comprising: 
 a voltage limiter which receives said MRI signal and produces a limited signal;    a filter electrically coupled to said voltage limiter, said filter designed to remove frequencies within said limited signal that are within a desired frequency bandwidth to produce a filtered, limited signal; and    a noise detection circuit which compares the filtered, limited signal to a reference noise signal and produces a first output if said filtered, limited signal is greater than the noise reference signal, and produces a second output different from said first output if said filtered, limited signal is less than said noise reference signal.    
     
     
         18 . The interference detector system of  claim 17  further including an automatic adjustment circuit that automatically adjusts the level of said reference noise signal.  
     
     
         19 . The interference detector system of  claim 18  wherein said automatic adjustment circuit automatically adjusts said reference noise signal based upon the statistical distribution of the filtered, limited signal.  
     
     
         20 . The interference detector system of  claim 19  wherein said automatic adjustment circuit automatically produces a reference noise signal at a level that is desired number of standard deviations away from the mean value of the limited, filtered signal.  
     
     
         21 . The interference detector system of  claim 17  further including a timer that determines if said limited, filtered signal exceeds said noise reference signal at least twice during a predetermined time period, wherein said noise detection circuit produces said first output only if said limited, filtered signal exceeds said noise reference signal at least twice during said predetermined time period.  
     
     
         22 . The interference detector system of  claim 21  wherein said predetermined time period is on the order of 100 nanoseconds.  
     
     
         23 . The interference detector system of  claim 21  further including a switch that blanks said signal when said noise detection circuit produces said first output.  
     
     
         24 . An interference detection system for detecting interference in a signal received from an MRI receiving antenna comprising: 
 a filter designed to remove frequencies within said signal that occur within a desired frequency bandwidth, said filter producing a filtered signal;    a comparator that compares said filtered signal to a noise reference signal; and    a detector that determines whether said filtered signal exceeds said noise reference signal at least twice within a predetermined time period.    
     
     
         25 . The interference detection system of  claim 24  further including a voltage limiter which limits the voltage of said signal prior to passing through said filter.  
     
     
         26 . The interference detection system of  claim 24  further including an automatic adjustment circuit that automatically adjusts the level of said reference noise signal.  
     
     
         27 . The interference detection system of  claim 26  wherein said automatic adjustment circuit automatically adjusts said reference noise signal based upon the statistical distribution of the filtered signal.  
     
     
         28 . The interference detection system of  claim 24  wherein said predetermined time period is on the order of 10-100 nanoseconds.  
     
     
         29 . The interference detection system of  claim 28  wherein said predetermined time period is set to be greater than twice the value of an expected frequency component of said transient interference.  
     
     
         30 . The interference detection system of  claim 25  further including an automatic adjustment circuit that automatically adjusts the level of said reference noise signal based on changes in the noise amplitude of said filtered signal.  
     
     
         31 . A method for detecting transient interference in a signal received from an MRI receiving antenna comprising: 
 filtering out from said MRI signal frequencies that are within a desired signal bandwidth to produce a filtered signal;    generating a reference noise signal that has a voltage that is a desired number of standard deviations away from a mean value of said filtered signal;    comparing said filtered signal to said reference noise signal; and    determining that said filtered signal likely includes transient interference if said filtered signal exceeds said reference noise signal at least once.    
     
     
         32 . The method of  claim 31  wherein determining that said filtered signal likely includes transient interference occurs only if said filtered signal exceeds said reference noise signal at least twice within a predetermined time period.  
     
     
         33 . The method of  claim 32  further including limiting the voltage of said MRI signal prior to filtering out the frequencies within the desired signal bandwidth.  
     
     
         34 . The method of  claim 31  wherein generating the reference noise signal further includes determining a voltage that has a desired deviation from the mean value of said filtered signal, and multiplying said voltage by a multiplier to produce said reference noise signal.  
     
     
         35 . A transient interference detector for detecting transient interference in a signal received from an MRI receiving antenna comprising: 
 a filter that filters out frequencies contained within said signal that are within a desired signal bandwidth, said filter producing a filtered signal;    an automatic noise adjustment circuit that generates a noise reference signal that has a desired probability of exceeding any thermal noise in said filtered signal; and    a comparator that compares said filtered signal to said noise reference signal and outputs a first signal if said filtered signal exceeds said noise reference signal and outputs a second signal if said noise reference signal exceeds said filtered signal.    
     
     
         36 . The transient interference detector of  claim 35  further including a voltage limiter which limits the voltage of said signal prior to passing through said filter.  
     
     
         37 . The transient interference detector of  claim 35  further including a timer that determines if said filtered signal exceeds said noise reference signal at least twice during a predetermined time period.